To understand language, children must learn to listen for meaning in sequences of speech sounds that unfold] rapidly in time. Fluent understanding requires "listening ahead," continually anticipating what is coming next by using linguistic and nonlinguistic information from the context in which words are spoken. Many studies have shown how skilled adults draw on multiple sources of knowledge to process speech in real time with remarkable speed and efficiency (Tanenhaus & Trueswell, 1995). Recent studies in our laboratory using the "looking-while-listening" task have documented the dramatic gains in speed and accuracy of speed processing abilities in English-language learners over the 2nd-3rd yrs of life (Fernald et al., 2001), and the robust links between efficiency of speech processing and individual differences in the growth in lexical an grammatical abilities (Perfors et al., under review). In addition, we have embarked on exciting crosslinguisti studies examining these online language skills in children learning Spanish as a first language. The studies proposed in this supplemental application extend this line of research to a longitudinal sample of children learning both English and Spanish at the same time. We will apply newly developed measures to examine language background in this important and growing population, as well as statistical techniques (HLM) that are particularly suited to analyses of longitudinal samples. The proposed project addresses three goals: (1) To document developmental increases in speed/accuracy of processing in children learning both English and Spanish and to compare these trajectories to those of participants in our ongoing study of monolingua Spanish-learners, matched on SES and vocabulary size. (2) To examine relations between spoken language processing and offline measures of vocabulary in bilingual children. Results could indicate that the relation: previously reported for monolinguals reflect representations or processing mechanisms that are shared in the context of constructing a particular body of language knowledge (e.g., English or Spanish). Alternatively, we might find that language-processing efficiency is more related to general measures of vocabulary, suggesting that processing efficiency is a stable characteristic of an individual that facilitates vocabulary growth across different language-learning contexts. (3) To address key theoretical issues about mechanisms of language acquisition that rely on strong associations between lexical and grammar learning (Bates & Goodman, 1997; Tomasello, 2003). Studies of children learning two languages are well-positioned to evaluate claims regarding the nature and specificity of the mechanisms guiding the development of language skills in young learners. [unreadable] [unreadable]